Metabolic syndrome (MetS) is a complex trait characterized by multiple abnormalities in glucose and fat metabolism, involving incompletely understood biological networks between various organs, and influenced by genetic and environmental (GXE) interactions. GPRC6A is a nutrient sensing G-protein coupled receptor implicated in the unique regulation of energy metabolism. In genetically engineered mouse models (GEMMs), GPRC6A regulates glucose and fat metabolism and prevents high fat diet (HFD) induced metabolic complications through direct tissue-specific effects and the release of hormones that coordinate metabolic functions between organs. The complexity of the cellular and systemic metabolic networks regulated by GPRC6A, the variable phenotypes in GEMMs, and the limited understanding of GPRC6A functions in humans are critical barriers to defining the role of GPRC6A in preventing and treating MetS and its complications. Our central hypothesis is that GXE interactions influence GPRC6A regulation of energy homeostasis. Aim 1 will test the hypothesis that GXE inter- actions modify GPRC6A regulation of glucose and fat metabolism in the liver and other metabolically active organs using GEMMs and a reductionist approach. Experiments will use wild-type GPRC6A-KGRKLP and GPRC6A null mice, HFD, and GPRC6A agonists to explore the effects of HFD and loss- and gain-of GPRC6A function on energy metabolism in mice. The functional significance of the recently evolved human GPRC6A_KGKY genetic polymorphism will be tested in a ?humanized? Gprc6a_KGKY_knockin mouse. We will characterize hepatocyte-specific Gprc6a knockout mice (Gprc6aliver-cko) to investigate GPRC6A?s function in liver, as a prototypic organ controlling glucose and fat metabolism. In Aim 2, we will use groundbreaking resources for systems genetics systems to test hypothesis that genetic backgrounds modify the metabolic effects of GPRC6A and HFD. We will collect metabolic phenotypes and molecular expression data from the livers of BXD recombinant inbred lines treated with a HFD and the GPRC6A agonist, osteocalcin (Ocn). Then we will apply systems biology approaches to define signaling pathways, metabolic processes and gene networks involving GPRC6A regulation of hepatic fat and glucose metabolism. Cell, molecular and mouse genetic approaches will validate these pathways and net- works predicted by systems biology. The predictive power of experimental and computational systems biology approaches to incorporate and integrate distinct levels of information and scientific knowledge of complex systems created by GPRC6A will improve the rigor and reproducibility of preclinical studies of GPRC6A effects on MetS. Our impact will be to: 1) establish the organ-specific functions of GPRC6AKGRKLP and GPRC6AKGKY variants and determine if these polymorphisms alter the susceptibility to and treatment responses of MetS and its metabolic complications; 2) identify the GPRC6A-regulated gene networks controlling glucose and fat metabolism and determine the genetic modifiers that influence the effects of GPRC6A and HFD on MetS; and 3) validate GPRC6A as a unique molecular target for understanding the pathogenesis and treatment of MetS.